[VMG]

These other cryptocurrencies are less secure than bitcoin.

For them, the cost of attacking the network by mining on an orphaned chain is lower, because by definition the PoW has a useful by-product.

The more useful the byproduct, the less secure the coin.

Edit: to illustrate, imagine a PoW that computes cures for cancer. Somebody can now mine on whichever block of the chain he wants forever, since he's producing a cancer cure anyway. The incentive of a block reward (within the currency) is not primary. Thus there is diminished incentive for the network to create a linear blockchain, where everybody mines on the most recent block. Without that, you cannot order transactions.

[bduerst]

A cure-cancer PoW where each block fixes all of cancer is ridiculous.

A more apt analogy would be that each block would solve a protein folding or active site matching problem for cancer. Then each solution would be public, and useless to mine over again.

[VMG]

It was hyperbole. It's the same deal in your example: if the is a PoW reward outside the currency, the cost of mining on the wrong chain drops.

The PoW must only be useful for the coin you are mining and nothing rise.

[bduerst]

Except the miners don't gain value from discovering a protein fit, so they have no incentive to mine the wrong chain.

Even then, as long as the coin value is larger you will have a consensus with the miners to to mine the latest block. Any sidechains will be neglected, which is why this "no value or nothing" attitude doesn't fit a blockchain network.

[VMG]

> Except the miners don't gain value from discovering a protein fit, so they have no incentive to mine the wrong chain.

Well then they are doing useless work too, and the criticism of wasteful mining applies. You gain value from doing useful things.

[bduerst]

Protein fit is only useless for the miners, but they have coin rewards on completion, so the work on new blocks isn't useless to them.

You do understand how a blockchain works, right?

[VMG]

You picked protein fit as an example of useful PoW. How can you now claim it's useless?